1. Field of the Invention
The present invention relates to a method and a system for avoiding cavitation in a pump conveying saturated water, said pump being connected to a saturated water source via a suction line. The invention relates, furthermore, to a thermal power station having such a system.
2. Discussion of Background
What is referred to as cavitation is the formation of steam bubbles and the decomposition of steam bubbles in a flowing liquid in the event of a change in pressure and/or velocity. If the pressure falls below the steam pressure of the liquid as a result of friction, due to the acceleration of a flowing liquid, etc., steam bubbles form in the liquid. If there is a subsequent pressure rise, for example as a result of a deceleration of the liquid, condensation of the steam bubbles takes place. On account of a sudden change in volume, this process is accompanied by very strong pressure pulses which may cause considerable damage, for example in a pump conveying this liquid.
At various points in thermal power stations, water in the state of saturation is conveyed, this being accompanied by an increase in pressure, and there is therefore the risk of cavitation.
The feed of regenerative preheaters from a feedwater tank/deaerator or condenser or the feed of an evaporator system from an evaporator, drum may be mentioned as examples.
Pressure fluctuations in the inflow system of the pumps of the abovementioned parts of a plant may lead to evaporation of this kind and therefore cavitation. Such pressure fluctuations may be caused, for example, by rapid load changes during the startup and shutdown of a thermal power station or by the sudden discharge of large steam quantities from the evaporator drum of a steam generator due to extraction or blowoff.
Pump damage caused by cavitation has hitherto been prevented by regulating the pressure in the inflow system, in particular by limiting the pressure drop on the suction side of the pumps and by monitoring the conveying behavior of the pump, including triggering a safety shutdown in the event of inadmissible conveying conditions.
Known devices for regulating the pressure in the inflow system cannot always, particularly in the event of brief malfunctions, ensure the necessary conditions for the pump to operate satisfactorily, and this may lead to a safety shutdown. However, a safety shutdown of pumps in a plant is undesirable.
The object of the invention is to provide a method and a system for avoiding cavitation in a pump conveying saturated water, said system ensuring that the pump continues to operate in the event of inadmissible pressure changes in the inflow system of the pump, for example due to a delayed, faulty or inadequate response of the means of regulating the pressure in the inflow system. A further object is to provide a thermal power station having such a system.
This is achieved, according to the invention, in that the actual values of the pressure and/or temperature of the saturated water in at least the saturated water source are detected and are compared with fixed pressure and/or temperature values which are conducive to cavitation, and, in the event that the actual values reach the fixed values, admixing water is supplied to the pump on the suction side, the temperature of said admixing water being lower than the fixed temperature value.
The system for carrying out the method is distinguished by an admixing water source which is connected to the suction line via an inflow line having an actuator. A valve, flap, slide and the like may be used as an actual form of construction of the actuator.
A thermal power station having such a system is distinguished in that the admixing water source is a water reservoir which is a water-containing tank or a water-containing system located upstream of the saturated water source in the throughflow direction of the water or steam circuit or arranged within the auxiliary systems of the power station.
The admixing water may be supplied, for example, by pumping, by sucking in the admixing water by means of a pressure difference prevailing between the admixing water source and the admixing point upstream of the pump, by means of a height difference between the admixing water source arranged in a plant and the admixing point upstream of the pump, or by a combination of these possibilities.
In a thermal power station having such a system, the admixing water source may be located upstream of the saturated water source in the throughflow direction of the water/steam circuit or be a low-temperature water reservoir arranged within the auxiliary systems of the power station, for example a water-containing tank, a water system, a water tank, a water line, etc.